1. Field of the Invention
The present invention relates generally to gas turbine engine, and more specifically to a turbine stator vane made from a high temperature resistant material.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
Most stator vanes used in the large frame heavy duty industrial gas turbine engines are made of a single piece from a casting process because of the low cost and high yields. One stage or row of stator vanes can cost over one million dollars. Thus, low casting yields (when a large number of the cast parts are defective) can be very expensive.
One way to improve the efficiency of the turbine is by forming the turbine airfoils from even higher temperature resistant materials so that a higher turbine inlet temperature can be used. To allow for higher temperatures, materials such as directionally solidified (DS) metals or single crystal (SC) metals have been proposed. However, forming a SC vane from a single piece is cost prohibitive because of the very low yields. A single crystal metal is formed by growing the crystal from one end of the vane to the opposite end. This works well when the metal is just a straight piece, but when the two endwalls must be formed integral with the airfoil is when the difficulty arises. The endwalls are formed at 90 degrees from the airfoil section and therefore the single crystal growth does not occur because of the directional change. To solve this problem, the stator vane is made from several pieces that can then be joined together.
Oxide dispersion strengthened (ODS) materials and directionally solidified eutectic (DSE) alloys are materials that are known for high creep life and high oxidation resistance. Several materials from these classes have creep and oxidation lives about three times those measured for conventional superalloys. ODS materials use mechanical techniques during processing to evenly distribute hard oxide particles of sizes less than about 0.1 micron within a metallic matrix, with the particles serving to make deformation of the material more difficult. DSE alloys are characterized by carefully controlled chemistry and processing, which produce a unique microstructure comprising the inherent fibrous or, in some cases, lamellar structure of the eutectic phase, with the fibers or lamellae aligned along a desired axis of the cast part in a manner analogous to a fiber-reinforced composite. DSE materials are also notable for excellent fatigue life, with certain alloys having about three times the fatigue lives measured for conventional superalloys. The careful processing control needed to produce ODS and DSE alloys cause these materials to be prohibitively expensive. ODS formed alloys exhibit creep rupture lives exceeding those of commonly used single-crystal superalloys by a factor in the range from about 2 to about 10, where the test load is about 21 MPa at a temperature of about 1150 degree C. The chromium in the alloys, present from about 15 weight % to about 20 weight %, provides effective oxidation resistance to the Ni-based matrix.